1. Field of the Invention
The invention relates to a method of producing highly branched polyesters with unsaturated end groups, which are useful in crosslinking systems.
2. Description of Related Art
Starting from the selective development of polymers containing no linear chains or segments but repeatedly branching from a central point out to the periphery of the macromolecule in regular patterns, i.e., what are known as dendrimers, synthetic products have been developed which give rise to highly branched polymers.
Unlike dendrimers, which have to be constructed from low molecular modules, the building principles of which were described by Tomalia et al. in Angew. Chem. Int. Ed., 29, 138 (1990) and Frechet in Science, 263, 1719 (1994) and in the J. Macromol. Sci., Pure Appl. Chem. A31 (11), 1627 (1994), highly branched polymers can be produced more easily and preferably in one- or two-stage reactions although they no longer possess the high regularity of branching in the structure of the macromolecule. Due to the fact that they are principally easier to produce, however, they are of more interest to industry than dendrimers. Dendrimers and xe2x80x9chighly branchedxe2x80x9d polymers are of interest because of certain unique properties which these new classes of polymers have. For example, the viscosities of solutions and melts of these types of polymer are significantly lower than those of linear random coil polymers with similar structural elements and comparable molecular weights because the highly branched macromolecules do not have a coiled conformation in solution or in the melt but a densely packed globular or spherical structure and therefore have a significantly smaller hydrodynamic radius than coiled macromolecules. In theory, therefore, it should not be possible for any entangling to occur between chains of different macromolecules in the melt or in concentrated solutions, which will increase viscosity. The same applies for mixtures of highly branched and linear polymers in solvents, where even a small proportion of highly branched polymers in the polymer mixture can significantly reduce the viscosity of the solution, as C. M. Nunoz et al. were able to demonstrate in Macromolecules, 33, 1720 (2000).
Reactive groups are essentially located at the outer ends of the chain branches, where they are available for other reactions. As the diameter of the macromolecule grows, the number of these groups also increases.
Highly branched polymers can be produced as described by A. Hult et al. in Macromolecules 28, 1698 (1995), for example, and in patent document WO 93/17060 by polycondensing a monomer with a group A and two same groups B or what is known as an AB2-monomer. A 2,2-dimethylol propionic acid is specified as a suitable monomer. According to patent document WO 93/17060, in order to obtain a higher degree of branching, a polyfunctional alcohol such as trimethylol propane or pentaerythritol is firstly condensed with this acid in a molar ratio of 1:3 and the resultant hexafunctional polyol is polycondensed with more 2,2-dimethylol propionic acid. The end groups of this polycondensate are predominantly OH groups.
Other methods are based on polycondensing a polyol with a cyclic anhydride, as described by Ranby and Shi in U.S. Pat. No. 5,834,118 or by polycondensing a dialkanolamine with a cyclic anhydride, as claimed in patent document NL 1008189.
For the purposes of many applications, it is of advantage to introduce reactive end groups other than OH or COOH groups. For example, it appears to be desirable to provide highly branched polymers with unsaturated end groups, such as acrylate or methacrylate groups, for example, in order to take advantage of the properties of this class of polymer to produce compositions which can also be polymerized and crosslinked by radicals, such as UV-curable lacquers and printing inks, fiber-reinforced polyester molded components, stereolithographically produced three-dimensional components or polystyrene with a high impact strength, for example. However, tests conducted by the inventors in an attempt to modify OH terminated highly branched polymers with acrylate and methacrylate end groups demonstrated that it is very difficult to transform the OH end groups with suitable unsaturated reaction partners as desired, because the modification is incomplete and can lead to degradation of the polymer or premature crosslinking.
The underlying objective of the invention is to provide a method of producing highly branched polyesters for use in crosslinking systems, which can be easily conducted on an industrial scale and is generally applicable.
In particular, the method must make it possible to obtain polyesters with unsaturated end groups in a single step, preferably with acrylate end groups, the reaction products having the greatest possible variability in terms of structure, enabling them to be used for different applications.
The objective of the invention is to propose a method of producing a highly branched polyester with unsaturated end groups, comprising polymerizing m-functional sorbic acid esters as component A and n-functional acrylic acid esters as component B in a Diels-Alder reaction at a temperature of from 50 to 150xc2x0 C., where
(i) m and n are whole numbers from 2 to 6 and the difference between m and n is at least 1 and
(ii) components A and B are used in a molar ratio which satisfies the following equation (1):
ne/nvxe2x89xa7fvxe2x88x921xe2x80x83xe2x80x83(1)
xe2x80x83
where ne is the molar number of the component with the lower functionality, nv is the molar number of the component with higher functionality and fv stands for the functionality of the component having a higher functionality.
Consequently, in accordance with the method proposed by the invention, polyfunctional sorbic acid esters (trans-2,5-hexadiene-1-carboxylic acid ester) are transformed with polyfunctional acrylic acid esters in a Diels-Alder reaction to obtain highly branched polyesters with isomeric 4-methylcyclohexene-2-dicarboxylic acid ester groups and unsaturated end groups.